The present invention relates to a communication method which is suitable for mobile communication systems, and specifically relates to a method suitable for use in measuring signal quality and controlling mobile stations in mobile communications wherein multiple-access connections between mobile stations and fixed stations are performed by a CDMA method.
Mobile communications have become widespread, and TDMA (time-division multiple access) is often employed as a multiple access method, but in recent years, the trend has been toward employment of CDMA (code-division multiple access) which has advantages such as making efficient use of frequencies, being able to readily handle communications at different transmission rates, and being difficult to intercept.
When a mobile station such as a cellular phone is on standby or is in communication on a radio channel set up by a certain base station, the signal quality on that radio channel will change when the mobile station moves.
Therefore, in order to attain radio channels with better quality, a mobile station performs as follows. Firstly, the mobile station receives radio channels that are transmitted from the neighboring base stations and also other radio channels that are transmitted from the base station which has already established a radio link with the mobile station, and measures the signal quality of those radio channels. Secondly, the mobile station compares the quality of those radio channels with that of the radio channel being used for standby or connection. Thirdly, the mobile station controls switching to a radio channel found to have high quality as a result of the comparison.
Here, with TDMA, multiple access connections are made by dividing the time of use of the same radio frequency among each user. That is, the radio channels are formed by dividing the time of use of a single radio frequency into a plurality of time slots, each user using a different time slot. In a mobile communication system employing TDMA in this way, time slots, which are not used by users, are specially provided aside from the time slots used for transmission and reception. During those idle slots, the signal quality of radio channels that are aside from the radio channel that has already been used to establish a radio link are measured. Then a comparison is made with the quality of the radio channel which was used for standby or communication, and the radio channel is switched to the one with the highest quality.
On the other hand, CDMA is realized by using different codes for each user on the same radio frequency. For this reason, the time of use on the same frequency is not divided for the sole purpose of achieving multiple access connections. There are no slots or frames reserved especially for measuring the quality of the radio channels.
Therefore, in mobile communication systems employing CDMA technique, in order to receive radio channels that are apart from the radio channel that has been used to established a radio link and measure the quality thereof, it is necessary to have two receivers, i.e. a receiver used for communications and a receiver for receiving other radio channels that are transmitted from the neighboring base stations as well as the same base station which has established a radio link with the mobile station, and measuring the quality thereof, as shown in FIG. 6.
Each receiver system in FIG. 6 principally comprises a frequency converter for converting a signal received in the antenna to a processing frequency band by mixing (multiplying) with a signal generated by a synthesizer 40, and two PSK detectors for QPSK (quadrature phase shift keying) detecting the down-converted signal. The signal received by the antenna is filtered by a filter 31, 41. The filtered signal is then mixed with the signal from the synthesizer 40 by the mixer 32, 42. The frequency of the signal from the synthesizer 40 (f1 or f2) to the mixer 32, 42 differs between the receiver system. In any case, the mixed signal is then supplied to another filter 33, 43 and the output from this filter 33, 43 is supplied to an automatic gain control (AGC) 34, 44. The signal supplied from the AGC 34, 44 is split. The split signals are then mixed at a pair of mixers 35, 37 and 45, 47. The split signals are mixed with a signal from the synthesizer 40 that has been divided into orthogonal phases by a phase splitter 36, 46. The output from each mixer in the pair of mixers 35, 37 and 45, 47 is then supplied to A/D converters 38, 39 and 48, 49 and the digitized output from the A/D converters, 38, 39 and 48, 49 supplied to the baseband portion 50. The carrier waves supplied from the synthesizer 40 to each PSK detector are mutually out of phase by xcfx80/2. Then, the baseband portion 50 performs communications with the base station by executing a designated procedure with respect to the signals received from the base station based on in-phase signals and orthogonal signals detected by the communication receiver, and measures the quality of the radio channel being used for communication. The baseband portion 50 also measures the quality of radio channels other than the radio channel currently being used to establish a radio link for receiving user information. The measurement is based on the demodulated in-phase and orthogonal signals received by a signal quality measuring receiver. Then, the baseband portion 50 compares the qualities of the radio channels measured above.
However, due to the demand for more compact and lightweight cellular phones, the structures such as to provide an additional receiver exclusively for quality measurements of radio channels in adjacent base stations goes against such demands. Additionally, it is important to reduce the power consumption because cellular phones basically use rechargeable batteries as power sources, structures which provide two receiver systems in the mobile stations also go against demands for lower power consumption.
On the other hand, a structure wherein the radio channel quality measurements and radio channel switching control is not performed on the mobile station side but on the base station side might also be considered. However, with this type of structure, each base station must separately provide a quality measurement receiver corresponding to each mobile station, and the processing load on the network side including the base station will increase, so that this would be an extremely unrealistic resolution for the present day situation in which many mobile stations can be expected to be used at the same time.
The present invention has been made in view of the above situation, and has a first objective of measuring the quality of nearby radio channels without providing an exclusive receiver on the mobile station side and ensuring compactness, lightness and reduced power consumption in mobile communications performing multiple access connections by means of a CDMA method.
Additionally, a second objective is to largely reduce the load on the base station side.
In order to achieve the above objectives, the invention is a communication method in a communication system having a first transmission device and a plurality of second transmission devices that are communicating with the first transmission device. The communication method being performed by each of the second transmission devices, comprises a step of intermittently receiving signals transmitted from the first transmission device or intermittently transmitting signals to the first transmission device.
A preferred embodiment is a communication method wherein the first transmission device and second transmission devices perform multiple access connections by means of a CDMA method. The first transmission device transmits control information for controlling the second transmission devices and user information to the second transmission devices using a radio channel divided into a plurality of slots. The communication method comprises a step of the second transmission devices detecting slots in which user information does not exist. Then using those slots period, which do not contain user information, the second transmission devices receive other radio channels that are transmitted from the first transmission device and also radio channels that are transmitted from the neighboring base stations. The second transmission devices will perform the measurement of the quality of the received signals. (refer to FIGS. 3A-3D)
According to this embodiment, it is possible to measure the quality of nearby radio channels without providing an exclusive receiver on the mobile station side and ensuring compactness, lightness and reduced power consumption in mobile communications performing multiple access connections by means of a CDMA method. Furthermore, since the communication quality is measured in the mobile station and the radio channel switching control for zone transfer and handover is performed under the supervision of the mobile stations, it is possible to largely decrease the load on the base station side.
Additionally, a further embodiment comprises an initial transmission step of transmitting signals from the second transmission device to the first transmission device at a transmission rate R1; a suspension step of suspending signal transmissions from the second transmission device to the first transmission device; and a second transmission step of transmitting signals from the second transmission device to the first transmission device at a transmission rate R2 higher than the transmission rate R1. (refer to FIGS. 10A-10E)
According to this embodiment, after the suspension step of suspending signal transmissions from the mobile station (second transmission device) to the base station (first transmission device) is performed, the second transmission step of transmitting signals from the mobile station to the base station at a high transmission rate R2 is performed, thereby preventing the loss of information when communications are interrupted for the reception of neighboring radio channels.